Wave transmission system



P 15, 1931- R. A. M|LL-ER 1,823,659

WAVE TRANSMISSION SYSTEM Filed Sept. 12, 1929 /NVEN7'0R R. A. MILL-ER A TTUFrNEY' Patented Sept. 15, 1931 Unites STATES.

PATENT OFFICE RQBERT A. MILLER, on PLAINFIELD, NEW JERSEY, ASSIGNOR 'roYnELI, 'rELEPHonE LABORATORIES, INCORPORATED, on NEW YORK, 1v. Y., aooEPoRATIoN OF NEW YQRK V i WAVE r sansinssrolv SYSTEM Application filed September 12, 1929; Serial no. 392,121. r

This invention relates to wave transmission systems and especially to electric wave transmission systems employing amplifiers or electric spacev discharge devices. v

Objects of the invention are to reduce deleterious noise effects in such systems and, specifically, to so control their gain or transmission efiiciency as to reduce the noise effects and avoid overloading the discharge devices or amplifiers.

i In one specific aspect the invention is a vacuum tube amplifier with two amplifying stages in tandem relation, and with an input, gain adjusting potentiometer and an interstage, gain adjusting potentiometer, and to insure that as far as practicable without danger of overloadingthe first stage any transmission loss produced by the otentiometers will occur in the second or interstage potentiometer so that effects oftube noise or other disturbance originating in or enteringthe system between the two potentiometers will be reduced, the potentiometers are so constructed and so mechanically connected that the first potentiometer has substantially its minimum loss setting or adjustment except when the second potentiometer has some adustment in which it gives a voltage reduction approximately equal to or at least como parable to the difference between the voltage applied to the second potentiometer as aconsequence of the voltage applied to the grid of the first tube and the latter voltage, and the load capacity of the first tube is made sufficiently great to avoid likelihood of the amplitudes of the waves impressed'on the grid of that tube beingso great as. to cause wave distortion in the tube. As indicated in G. Green Patent 1,607,980, November 23, 1926, in amplifiers with input and interstage potentiometers or gain adjusting devices, as far as merely avoiding possibility of overloading the first stage is concerned, the first potentiometer'rather than the second ordinarily is the preferable potentiometer for introducing tra nsmission loss, in systems where the waves fed to the amplifier may attimes have such amplitudes as to cause overloading of the first stage unless the'first potentiometer is set to introduce suflicient loss at those times- However, where tube noise becomes objectionable, as for example because of the typeof tubes or tube energization employed or conditions of acoustic feed back or mechanical vibration under which'the tubes are operated, it may become advisable to provide for the reduction of the tube noise, and this can be done as indicated above without producing likelihood of overloading. In order that the portion of the total potentiometer loss that occurs in the second potentiometer before substantial-loss occurs in the first potentiometer may be large without danger of overloading, the load capacity of'the fload capacity as used 'in the specification and claims, refers to the amplitude of the input waves that can be applied to the tube without causing the tube to produce substantial distortion in the waves that it transmits.

The noise reduction in accordance with the principles of the invention is of special utility in' rendering practicable amplifiers of high gain for Waves of very low energy level Where the amplifiers require a wide range of volume control or gain variation and have in the first stage a heat'entype or uni-potential cathode type of tube that employs alternating current for heating its cathode. i

For simplicity in-explaining the principles of the invention, particular reference has been made to two stages of amplification and an input potentiometer and an interstage potentiometer. 'However, the application of the invention and the scope of thecl'aims'are not restricted to the control of two potentiometersor gainadjusting devices in the manner described but extend to, control in such manenrof any desirednumber of potentiometers or gain adjusting devices in any desired positions with respect to theamplim fiersor space discharge tubes ofia circuit.

first stage'may be made comparable to or I '7 and- A& in push-pull connection. Two

For example, three like potentiometer-s may be operated by a single shaft and may be respectively connected 1n the input circuits of the first three stages of avacuum tube amplifier and may respectively control the voltages applied to the grids of the vacuum tubes in those stages, the movement of the shaft to reduce the amplifier gain first causing the potentiometer in the input circuit of the vacuum tube in the third stage to attenuate the voltage to be applied to the grid of I that tube until the setting of substantially maximum attenuation for that potentiometer has been reached, and then causing the potentiometer in the inputcircuit of the vacuum I .WlllbG apparent from the following descrip- 1 'The amplifier shown tion' and claims.

The sin le fi ure of the'drawino is a dia- D O a gram of afvacuum tube amplifier embodymg the invention.

plifier, suitable for use'in, for exaniple, public address systems or sound reproducing systems employed for producing synchroniz'ed or non-synchronized sound pictures The" first two stages comprise uni-potential cathode tubes; A- -l and A'.-2:res 3ectively.

and the third stage comprises two tubes l t- 3 tubes R-'5 and R 6 are used for rectifying current obtained through a voltage step up transformer I from a 110 volt,760 cycle power supply circuit 10. The rectifiedcurrent is used to supply the grid and plate potentials for all stages. The currents for heating the filaments of all five of the tubes are alternating currents obtained from a power supply circuit 11 through voltage step down transformer 12. The tubes Al and A2 may be for example of the heater tyne disclosed in Freeman and Wade British Patent 209,415 assigned to the Westinghouse Electrio and' Manufaoturing 00., or the type disclosed in French Patent 593,321, Mayp22,

"1925 or the type disclosed in U. SpPatent to M. J. Kelly, 1,678,145", July 2 1928.

WVaves to be amplified are impressed on input transformer 16 from amplifier input circuit.l5. 'They may be, for example, Waves corresponding to sounds to accompany motion pictures either synchron zed or not synchronized with said sounds. j The waves are transmitted through that transformer to is a three stage aman amplifier input potentiometer or adjustable gain controlling attenuator 20 in the input circuit of tube A1, thence through that tube and an interstage transformer 21 to an interstage potentiometer 22, and thence through tube n+2 interstage transformer 25, tubes A'.3 and A-4, and output transformer 2 to amplifier output circuit 30.

The currents rectified by tubes R5 and R-6 pass through a filter 35, comprising inductance coil 36 as a series arm and condensers 37 and 38 as shuntarms, for suppressing voltage and current fluctuations. 'Points 40 and l1 maybe regarded as the positive and negative output terminals respectively of the filter 35. I A potential from which plate potential for tubes A-1 and A2 is obtained is derived from the junction point 42 of voltage dividingresistances a3 and ist, which are connected across points lO and ll by a filter consisting of an inductance coil 45 as a series arm anda condenser 4:6 as'a shunt arm. This filter assists the filter 3 5 in suppressing voltage and current fluctuations. The steady plate current for tube A'l passes from point 42 through resistances 4:7 and L8 to the plate of the tube, thence to the grounded cathode of the tube, and thence through resistances 51 and 52 to point ll. The steady plate current for tube A2 passes from point 42 through resistances 53 and 54Ev to the plate of the tube, thence to the/grounded cathode, and thence through the resistances 51 and 52 to the point d1. a V i Thesteady plate current for tubes A-3 and A-4 passes from the point 40 through a choke coil 56 for suppressing current fluctuations the primary windings of transformer 26, the tubes A-3 and 'A%, the middle pair of secondary windings of the transformer 12, and resistances 51 and 52, to the point 41. 7 Negative grid biasing potential for the tubes A.1 and A-2 is obtained from the voltage drop in resistance 51. The biasing potential for the grid oftube A1 is applied from the junction of resistances 51 and 52 through a resistance and condenser filter 60 and the secondary Winding of transformer 16 and the potentiometer 20. The filter 60 consists of resistances 61 and 62 as series arms and condensers 63 and 65t as shunt arms.

The biasing potential for the grid of tube A-2' is applied from the junctionof resistances 51 and 52 through a filtering series resistance and shunt condenser 65 and 66 and the secondary winding oftransformer 21 and the potentiometer 22,

Negative grid biasing potential for the tubes A-3 and ,A-4 is obtained from the voltage drop inresistances 51 and 52. The biasing potential is applied from point 41 through a filtering series resistance 68 and shunt condenser 69 and the secondary Wind ings of transformer 25.

i a an Condensers 71 and 72 are the usual stopping condensers, of negligiblylow reactance for the frequencies of the'wavetobe amplified, for blocking passage ofdirectcurrent.

Fluctuations of the plate current supply for tube A1 are further prevented by a filter consisting of the resistances 47 and 48 as series arms and condensers 73-and 74 as shunt arms. Fluctuations of the plate current supply for: tube A-2 are-further prevented by a filterconsisting of the resistances 53 and 54: as series arms and-condensers- 73 and 75 as shunt arms. it i The volume control potentiometers 20' and 22 that control the amplitudes of the waves to be applied to the grids of the first two tubeshave their movable contacts and 82 mounted on the same shaft or-movable element The potentiometer 20 has fixed contacts which the contact 80 successively engages in its movement andwhich are con nected by resistances 91. The potentiometer 22 has fixed contacts 92 which the contact 82 successively engages in its movement and which are connected by resistances 93. The contacts 90 are so'located relatively to the contacts 92 in the direction of motion of the contacts 80 and 82, that as the shaft 83 is rotated or moved to move the contacts 80 and 82 from their position-of minimum loss in the potentiometers in order to reduce the gain of the amplifier, during the first part of the travel of contacts 80 and 82, the con-l tact 82engages contacts 92 successively and causes the loss introduced by potentiometer 22 to increase step by step to its maximum value While the loss introduced by potentiometer 20 remains at its minimum value, and

during the remainder of the travel'of' con- Y tacts 80 and 82 to decrease theamplifier gain, the contact 80 engages the contact 90 and causes the loss introduced by potentiometer 20 to increase step by step to its maximum value while thefloss introduced by potentiometer 22 remains at its maximum value.

When the contacts 80 and ,82 are moved in the opposite sense,vto increase the amplifiergain, the loss in potentiometer 20 is reduced to its minimum value before the loss in potentiometer 22 is changed from its maximum value. Attenuation occurring in poten tiometer 22, at any time, is not only attenuation of the signal and noisewaves applied to the amplifier from circuit 15, but is also attenuation of any noise entering or generated in the system at anyportionof the system preceding thepotentiometer 22, notably the noise introduced by tube A1. Thus, the attenuation which takes place in the grid cir- 1 cuit of the second tube affects equallythe signal and the noise generated orpicked up inthe first stag'e.- Such additional attenuation as may be necessary over an'dabove that obtainable in the secondpotentiometer is obtained in the first-potentiometer; 1

stationary contacts 94: all atthe same poten tial,.one positioned opposite each of the contacts92, and potentiometer 22 can include stationary contacts 96 all .at the same potential, one positioned opposite 'each of-the contacts 90, so that contacts 80 and 82 'caneach bear on a fixed contact in each setting of'the potentiometer. v

The voltage reduction introduced by potentiometer 22 before potentiometer .20 is moved fromits minimum attenuation setting can be made sufiiciently great to prevent'overloading tube A2 by operation of tube A-l .at its maximum load capacity. lVhen these tubes have the-same load capacity, this voltage reduction can be made substantially equal to or at least comparable to the difierence between the voltage applied to the potentiometer 22 as aconsequence of the voltage applied to the grid of tube oA--1c and the latter voltage. i-In other words, when these tubes have the same load capacity, the voltage attenuation inserted by potentiometer 22 before" potentiometer 20 is moved from its minimum attenuation setting can be made substantially equal to or at least comparable to thevoltage gain thatthe circuit gives between thegrid of tube A1 and the-input side of the potentiometer 22. In orderthat the attenuation introduced by potentiometer 22 before the potentiometer 20 ismoved from its minimum losssettingmay be madelarge without overloading tube A.1, the load capacity-of thattube-may be made comparable of the succeeding tube. Then the voltageattenuation. inserted by potentiometer 22 be- 7 fore potentiometer 20 is moved from its minimum attenuation setting can be made sub stantially greater thanthe voltage gain that the circuit gives between the grid oftube A1 and the input side of the potentiometer 22. -e

'7 Ifdesired, the first stage can be a pushpull stage (having tubes of the uni-potential cathode type, or other type) in order to reduce (inthe general manner indicated in D. (it. Blattner Patent 1,483,273, Feb. 12, 1924) noise resulting for example from the usefof alternating current to energize the heater elements or the filaments of the tubes of that stage. Then if the tubes used in the first twostages are of the same kind or of equal load capacity, the load capacity of the w first stage (i. e. the magnitude of the voltage that can be applied between its girds without e causingit to distort the signal) will be larger than the load capacity of the second stage;

and the voltage attenuation inserted by pomum loadcapacity. 1

put side of the potentiometer 22, or. in other words, canbe made sufficientlygreat to prevent overloading tube A-LQ by operation of the preceding push-pull stage at its maxi- In one system embdoying in the drawing, the tubes A1 and A2 are Western Electric Company, Inc, Type 247A vacuum tubes operated Withthe same plate potential (90 volts) and with substantially the same negative grid biasing potential (3 volts), the tubes A3 and A4 being Type 205D\ tubes of the same company. i The gain of the amplifier is about 84 decibels,

the gains of the first, second and third stages being approximately db., 30'dbsand db.

respectively. In that particular amplifier the maximum attenuation of potentiometer 22 is 30 db. and the'maximum attenuation of potentiometer 20vis 36 db. These'numerical values-are given merely by way of example, and the-invention is not to be regarded as limited thereby.

tainable in that stage.

. 2. Use is made of the large load carrying capacity of the initial tubes to keep the ratio 5 of signal to noise constant. ;This excess load carrying capacity in the initial stages results fromthe fact that in an "amplifier it is, in general, advantageous to use'as many vacuum tubes as possibl'eof the given kind, 'and'to operate these tubes at like B and C potentials. Afterfladvantage has-been takenjof any excess load carrying capacity of a given stage or stages preceding a given potentiometer, any further attenuation desiredis obtained. in the stages preceding the given potentiometer. V

34A high gain amplifier may be given a high-range volume control and so be used at low gain without increase in the ratio of noise to signal. This is because the gain in each stage is approximately counteracted by the attenuation in the succeeding stage.

hat isclaimed is: j I 1. A circuit comprising two adjustable transmission efiiciency controllers, andmeans connecting them in successive relationandintroducing disturbing waves in the circuit between them, .and means interlocking said controllers for insuring that the first controller is'set for substantially its maximum transmission elficiency "except when the second controller is in-a setting in which it ren-i ders its output voltage at least substantially as low as the output voltageofthe first 'con- ""5 troller.

the invention and hav ng the circuit configuration given 2. A signal transmitting circuit comprising an electric space discharge device, an adjustable transmission efficiency controller feeding signaling waves to said device, a translating device, a second transmission elliciency, controller for transmitting signaling Waves from said first device to said translating device, and means interlocking said controllersfor insuring that the first controller is set for substantially its maximum transmission efliciency except when the second controller is in a setting in which it prevents thetransmission level of the Waves thatit feeds to said translating device from being so great as to cause the latter device to produce distortion of the signal when the trans mission level of the Waves fed to said first device from said first controller is substantially as high as the maximum transmission level that said first device can receive from said first controller without producing distrollers for insuring that the first controller is set for substantially its maximum trans mission efficiency except when the second controller is in a setting in which it prevents the transmission level of the wavesthat it feeds to said translating device from being so grea as to cause the latterdevice to produce distort-ion of the signal when thetransmission level of the Waves fedto said first device from said first controller is at least substantially as high as the maximum transmission level that the second device can receive from said second controller without producing distor tion of the signal in said translating device.

4. A circuit comprising an electricspace v discharge amplifying device having an anode,

cathode and a discharge control element, an input circuit for said device, an adjustable voltage attenuator in said input circuit, an adjustable voltage attenuator having input terminals fed from said device, and means interlocking said attenuators for insuring that the first attenuator is set for substantially its minimum attenuation except when the second controller is set to give a voltage attenuation a at least of the order of magnitude of the voltage amplification from said discharge control element and cathode of said first tube to the input terminals of said second voltage attenuator. Y

5. A circuit comprising two adj ust-able transmission efiicienc-y controllers and means connecting them in tandem relation and introducing disturbing waves in the circuit between them, and, means, interlocking said controllers for insuring that, as far as practicable Without overloading said first means, any decrease in transmission efficiency produced by said controllers occurs in the controller fed from said first means, said first justment except When the second potentiometer has substantially its maximum loss adjustment, the maximum loss of the second potentiometer being as great as practicable Without overloading the first stage prior to overloading succeeding stages, and the load capacity of the first stage being at least substantially as great as that of the second stage.

7. A circuit comprising two tandem connected stages of electric space discharge amplifying devices each having an input circuit and a discharge control element, the load capacity of the first of said devices being at least as great as that of the second, adjustable transmission eiiic-iency controllers, one in the input circuit of each of said devices, and

out causing distortion in the first device and the maximum voltage that the discharge control element of the second device can stand without causing distortion in the second device.

In Witness whereof, I hereunto subscribe my name this 6th day of September, 1929.

ROBERT A. MILLER. 

